nzyme catalysed reactions can occur at significant rates at low temperatures, thus exerting a major control on the quality of frozen foods. The temperature-concentration effect in partially frozen systems can make enzyme reactions controlled by diffusion and thus rate limited by viscosity according to the Stokes-Einstein relation. Polyphenoloxidase (PPO) activity in sucrose, glycerol and fructose water media and (POD) activity in fructose and glycerol water media, were assayed spectrophotometrically from 20°C to -30°C. The media have at equal sub-freezing temperature marked differences in viscosities and others physical properties. For both enzymes, the activity data showed similar dependence on temperature and, above freezing, on the concentration of the media. A relation with the viscosity of the media at equal sub-freezing temperature was not evident. POD activity showed a 'break' between -10°C and -20°C, suggesting a reversible conformational change. The temperature dependence of PPO rate constants were well described by both the Williams-Landel-Ferry (r 2 = 0.998) and the Arrhenius (r2 = 0.985) models. A hypothesis for the non conformity of the measured kinetics with the theoretical model is that in the temperature range considered the reactions are still activation controlled or in a transition zone, and should become diffusion limited only at lower temperatures and higher viscosities.
